We studied the effect of the secondary metabolite emodin on food intake, food assimilation mass coefficient (AMC), feeding bout rate, and defecation rate in a frugivorous bird, the Yellow-vented Bulbul (Pycnonotus xanthopygos). Emodin is found in the ripe fruits of Mediterranean buckthorn (Rhamnus alaternus), which is commonly eaten by P. xanthopygos. Emodin (0.005 and 0.01% wet mass) increased dry matter AMC by 8–10% after isolating the effect of food intake. At a concentration of 0.001%, emodin increased the AMC of nitrogen, fat, and organic remains (mainly carbohydrates and proteins). Apart from emodin, fruits of R. alaternus contain a variety of secondary metabolites that may interact with each other and influence bird digestion. Artificial food laced with crude Rhamnus fruit extract increased food intake and dry matter AMC. Emodin (0.01% wet mass) increased the average time between defecations, but did not affect the time interval between feeding bouts nor the average amount consumed per feeding bout. We speculate that emodin increases food retention time. Longer retention time may explain the observed increase in AMC. Our results suggest that sometimes secondary metabolites in ripe fruit may not be detrimental to frugivores and the presence of emodin in the pulp of ripe buckthorn fruits might be selectively adaptive to the plant.